Field of the Invention
The present invention relates to films based upon or containing oriented dye. It also relates to methods of manufacturing such films and to polarizers and liquid crystal display devices utilizing the films. More specifically, the invention pertains to films made of or containing highly oriented dichroic dye, methods of manufacturing such films, and high-performance polarizers and liquid crystal display devices utilizing the films.
Description of Related Art
Polarizers are used as elements of liquid crystal displays (hereinafter referred to as LCDs). Polarizers have the property of absorbing some of the light transmitted from the LCDs. This becomes a particularly acute problem with full color LCD's since full color LCD's contain color filters as well. The polarizer present in the LCD and the color filter used in the full-color LCDs both absorb light. This reduces the quantity of transmitted light to a level that is often quite low. A powerful backlight is thereby required for sufficiently bright LCDs. This backlight can undesirably raise the temperature of the liquid crystal cell and increase the power consumed. One type of polarizer is manufactured by adsorbing iodine onto a uniaxially stretched, oriented alignment film of polyvinyl alcohol (hereinafter referred to as PVA) or a derivative of PVA.
A second type of polarizer is based on dichroic dyes and is similarly produced by adsorbing the dye onto a uniaxially stretched oriented PVA or PVA derivative alignment film. (Throughout this specification and claims, the terms "dichroic" and "dichroic dye" have their commonly accepted meanings and refer to the property of presenting different optical properties e.g., color--when viewed from different directions, and thus to in effect give a polarized response.)
A third type of polarizer is produced by uniaxially stretching a polyvinyl chloride or PVA film and then generating polyene units as chromophores by dehydrohalogenation of the polyvinyl chloride or by dehydration of the PVA.
The iodine-containing polarizers initially exhibit excellent polarizing power. These polarizers, however, have poor resistance to water and heat and are not sufficiently durable under conditions of high temperature and high humidity. Protective films or other possible measures typically do not significantly improve the durability of the iodine-containing polarizers.
The dichroic dye polarizers have greater resistance to water and heat than the iodine-based materials, but have inadequate polarizing properties for most commercial applications. Pigments, which have excellent weather resistance, cannot be substituted since they are not highly oriented in polymer films and thus cannot be used for polarizers.
In addition, many difficulties arise when trying to manufacture fine polarizers such as polarizers having a width of not greater than about 200 micrometers. Cutting a dyed alignment film is the only way presently used to prepare such fine polarizers.
Thus, the known methods of orienting dye molecules include the use of an oriented PVA film which aligns the dye molecules as has been described above. It is also known that one can mix a dye with a liquid crystal material and orient the dye as the liquid crystal molecules in the liquid crystal cell are themselves aligned. In the latter method, a liquid crystal material must be sealed between two glass plates or placed within a cell. This method further requires that an alignment film be formed on the surface of the glass plates. The degree of alignment of the dye depends upon the degree of the alignment of the liquid crystal material, so that in many cases the polarizer does not have a sufficient polarizing power due to low alignment of the liquid crystal material.
Films containing highly oriented dichroic dyes and particularly films which include micropatterned, highly oriented dichroic dyes and their easy manufacture are thus in high demand.
Although not relating to orienting dye molecules, J. C. Wittmann et al. have found that an oriented polytetrafluoroethylene (hereinafter referred to as PTFE) thin film may be formed by rubbing PTFE while applying heat and pressure. They have reported that alkanes, liquid crystal molecules, polymers, oligomers, and inorganic salts can be oriented using this oriented PTFE film as an alignment layer (See Nature, Vol. 352, p. 414 (1991) and U.S. Pat. No. 5,180,470). While alignment of these organic molecules with this oriented PTFE thin film has been reported in detail, alignment of dye molecules useful for polarizers, or more specifically, alignment of dichroic dye molecules for polarizers, has not been elucidated.